Moon structure. Cutaway computer artwork showing the internal layers that make up the structure of the Moon. The Moon has a diameter of 3476 kilometres. The outermost layer, known as the Regolith, is a covering of loose, heterogeneous material overlying solid rock. It includes dust, soil, broken rock, and other related materials. Under this is the crust (brown) overlying the mantle (orange). At centre is a small solid inner core (sphere, centre) surrounded by a fluid outer core (bronze, centre). The Moon is geologically dead. As it has no atmosphere or geological activity, its surface is still marked by craters from meteorite impacts from the last four billion years.